Victor Xie, --whether it is also OK to say "How is the weather like? The correct idiomatic expression is "What is the weather like?" If you ask someone "How is the weather?", they could answer, "Fine". This doesn't give you much information. If someone asks "What is the weather like?", they are asking what sort of weather it is. The response would be sunny, rainy, cold, snowy, etc. An analogy would be the question, "How is he?". To this you would expect a response such as "He is fine". On the other hand if you ask "What is he like?", you expect an answer such as "He is a friendly person.", or "He is an evil man."

We would never say 'how is the weather like?'

No, you can't say that.

Not really I would use the other two :)

when are you leaving/ when do you leaving?